Heart valve prosthesis

ABSTRACT

A frame system for heart valve prosthesis. A first frame has at least three generally parallel legs each comprising a pair of rod portions connected at one end and diverging at the other end as lobes respectively connecting with rod portions of others of said legs. The lobes form an aperture therebetween and the legs are adapted to receive the margin of a valve leaflet between the rod portions thereof. Thus, the leaflet may be secured to two adjacent legs and the interconnecting lobe so as to have a free edge extending between said adjacent legs. A second frame is adapted to nest with the first frame and comprises a rod formed to be substantially congruent with the interconnecting lobes, whereby the leaflets may be secured to and between the frames.

BACKGROUND OF THE INVENTION

The invention described herein was made in the course of work under agrant or award from the Department of Health, Education and Welfare.

This application is a continuation-in part of copending application Ser.No. 790,442, filed Apr. 25, 1977 by Robert B. Davis, John Skelton,Richard E. Clark and Wilbur M. Swanson, now U.S. Pat. No. 4,192,020,which is a continuation-in-part of abandoned application Ser. No.771,359 filed Feb. 23, 1977 by Richard E. Clark, John Skelton and RobertE. Davis, said abandoned application being a continuation-in-part ofSer. No. 575,438, filed May 7, 1975 and now abandoned.

This invention relates generally to sheet materials for vascular, heartvalve and other prosthetic implants. More particularly, it relates tosuch sheets fabricated of synthetic materials.

A principal object of this invention is to provide synthetic sheetmaterials having properties in use that closely approximate those of thenatural tissues that they replace. For an understanding of theseproperties the human aortic valve may be taken as an example, in that ittypifies properties that are required for other types of implants aswell, such as vascular implants. This valve is of the leaflet type,having thin flexible membranes with a face to face thickness of about0.06 cm that open 70 to 90 degrees from horizontal into the surroundingblood vessel (ascending thoracic aorta), and form three contiguouspouches held in close and leak-proof mutual contact by the pressure ofthe blood when in the closed configuration. Thus the membranes cause aminimum of disturbance to the flowing blood when in the openconfiguration, but move quickly when the blood pressure reverses(changes sign) to prevent regurgitation. A number of properties of thehuman valve may be identified, and these comprise the more specificobjects of the present invention.

A first characteristic of the human aortic valve is that its responsetime is minimal. Thus it is an object of this invention to provide asynthetic sheet material that has a low resistance to motion of theleaflets in terms of both the inertial and elastic components of suchresistance. In general terms, this is achieved by making the leaflets aslight in weight and as flexible as possible consistent with the othermechanical requirements of the valve discussed below. This will enablethe valve to pass quickly from the fully open state to the fully closedstate when the pressure differential changes sign, leading to reducedenergy losses in the flowing blood, and a minimum of undesirableregurgitation.

A second property of the human aortic valve is the effectiveness of theseal. Observations of this function reveal that the sealing of the valveis accomplished by the intimate conjugation of regions (referred to asthe coaptation zone) close to the free edges of the valve leaflets. Theeffectiveness of the seal depends upon the degree of compliance of theleaflet in directions both parallel to its plane and transverse to thatplane. The high transverse compliance allows the two contacting surfacesto form a more intimate conformal fitting at the coaptation zone, andthe high in-plane compliance insures that the coaptation zone issufficiently large to make an effective seal. Experience with syntheticvalve leaflet implants has shown that these two forms of compliance areinfluenced by different factors. When such implants are employed,natural tissue is deposited or formed on the leaflet, and the nature ofthis natural tissue depends upon the nature and geometry of thesynthetic material in use. The transverse compliance is controlled to alarge extent by the mechanical properties of this tissue. In contrast,the in-plane compliance is controlled directly by the mechanicalproperties of the substrate synthetic material. Therefore, for suitablein-plane compliance it is an object of this invention to providesynthetic materials having longitudinal (i.e., tensile) compliancesimilar to that of the human leaflet. In addition, for adequatetransverse compliance, additional criteria must be met with respect tothe promotion of suitable natural tissue on the surface of the implantfabric.

A third property of the human aortic valve is the provision of adequateload bearing capacity. For a more detailed understanding of thisproperty, a distinction may be made between the tensile compliance inthe circumferential direction of the cusp, that is, the in-planedirection parallel to the free edge of the valve leaflet, and thetensile compliance in the radial direction of the cusp, that is, thein-plane direction perpendicular to the free edge. In either direction,the maximum working load level may be taken for practical purposes asapproximately 150 gm/cm of leaflet width. This load exists in the closedconfiguration of the valve during the peak operating pressure in thearterial system. These requirements for adequate load bearing capacitywould appear to impose properties different from those associated withthe high compliance requirements previously described. However, theparadox is resolved in nature by the marked nonlinearity of thestress-strain characteristic of the natural leaflet material. At lowloads the material has an extremely low modulus, thus ensuring bothrapid response and good conformal fitting, but at a particular value ofelongation (typically in the 10% to 20% region) the modulus undergoes amarked increase, with the result that the natural tissue can sustainhigh levels of loading without excessive subsequent elongation, grossgeometric distortion or rupture.

Quantitatively, the foregoing features may be described as follows. Inthe circumferential direction the natural leaflet extends very readilywith increased load until an elongation of 10% to 12% is reached at aload of 1 to 2 gm/cm of leaflet width. Upon further increase in theload, the resistance to further elongation increases greatly, and at themaximum working load level of 150 gm/cm width the modulus isapproximately 3600 gm/cm width, which corresponds to a modulus expressedin classical engineering units of 850 pounds per square inch (p.s.i.).In the radial direction of the cusp, the region of easy extension withincreased load continues up to approximately 20% elongation, at whichthe load is about two gm/cm width. Upon further increases in load theresistance to further elongation, though greater than in the initialregion, is not as high as it is in the circumferential direction. Theworking load in this direction is not as firmly established as in thecircumferential direction, but at a load of 150 gm/cm width the modulusis approximately 1000 gm/cm width (250 p.s.i.). Thus in each directionthere is a transition between an initial region in which the modulus isof the order of 10 gm/cm width and the high load region in which themodulus is between 1000 and 3600 gm/cm width.

In view of the variations that occur in the tensile moduli of naturalheart valve and vascular tissues, it is difficult to ascribe exactsignificance to the absolute values of the moduli described above.However, it appears that satisfactory performance of a syntheticprosthesis can be obtained if a modulus can be achieved which is withina factor of two of the natural leaflet values given above. Accordingly,it is preferred that the fabrics of this invention have a tensilemodulus that is no greater than about 7200 gm/cm of leaflet width norless than about 500 gm/cm of leaflet width up to a load of about 150gm/cm of leaflet width.

In copending application Ser. No. 901,085, now U.S. Pat. No. 4,191,218,the property of nonlinearity in the natural valve leaflet material isgenerally described, and a synthetic leaflet material composed ofmultifilament polyester yarns is disclosed. To obtain the desirednonlinearity in the stress-strain characteristic, these yarns aresubjected to compaction and shrinking to produce crimps in the yarns.The easy compliance of the crimped yarns at low stress levels resultsfrom the fact that the crimps are being straightened out; and thesubstantially higher tensile modulus exhibited at higher stressesresults from the fact that the straightened yarns are being stretched.

Another property of the natural valve leaflet is its ability to maintainits original geometry and structural integrity through a large number ofcycles of stress. For these purposes four times 10⁹ cycles may be takenas the pertinent criterion. Accordingly, it is an object of thisinvention to provide a sheet material that has a rapid and near-completerecovery from applied stress. The fatigue lifetime of a material that issubjected to cyclic stress and strain is influenced by the amount ofnon-recovered work (hysteresis loss) that is associated with the stresscycle. If there is rapid and substantially complete elastic recovery ofthe material from a cyclically applied stress, a longer fatigue lifetimewill generally result. Further, it is desirable to provide a sheetmaterial in which the applied stresses do not undergo a change ofdirection, as such a change tends to reduce the fatigue lifetime of mostmaterials.

In addition to providing properties similar to those of natural heartvalve leaflets, synthetic leaflets, vascular implants and the like mustbe so structured as to promote desirable tissue overlay when implanted,and the materials used must have adequate resistance to chemical changein the implanted environment.

SUMMARY OF THE INVENTION

According to this invention, sheet materials for cardiovascular andother prosthetic implants are fabricated of synthetic elastomers. Theseelastomers, when formed with appropriate geometrical configurations,exhibit all of the properties of the natural heart valve tissuepreviously described to a higher degree than those materials hithertoused. As used herein, elastomers are defined as polymeric materials thatexhibit rubber-like elasticity characterized by low modulus (highcompliance) and hence considerable elongation under low loads, withultimate elongation reaching up to 1000% in some cases, with rapidrecovery toward the original, unstretched dimension on removal of thestress, and with little or no permanent deformation as a result of theimposition of the stress. In polymeric materials of high molecularweight, this type of behavior is associated with a relatively smallnumber of permanent crosslinks, and with rubber/glass transitiontemperatures considerably below the operating temperatures. The elasticextension in these materials is associated with the change in theconfiguration of the long polymer molecules from a randon coil in theunstretched state to the extended condition. The resistance to such achange increases as the molecules become increasingly aligned with thedirection of stress, and the stress-strain curve usually shows anincrease in slope as the elongation is increased. This is the type ofelastic response that is needed to correspond with the response of thenatural valve.

There are certain differences between the elastomers of this inventionand the crimped fibers described in the abovementioned copendingapplication. In the elastomers of this invention the nonlinearity in thestress-strain characteristic is produced at the molecular level;whereas, in the crimped fabrics described in said application thenonlinearity is imposed at the supramolecular, that is the fiber, level,being associated with the bending deformation of the crimped fiber. Theprocess of unbending and rebending of the crimped filaments in the yarnsis associated with cyclical stress reversals. These stress reversals dueto this bending do not occur in the elastomers of this invention;therefore, an improvement in mechanical fatigue characteristics and alonger useful lifetime are achieved. Moreover, the elastomers of thisinvention differ from the polyester of said application in the highermodulus portion of the stress-strain curve. The differences here areexhibited by the relatively more rapid and more complete recovery of theelastomers when the stress is removed.

To achieve the desired minimal inertial and elastic resistance of thesynthetic sheet material to the motions introduced by the hydrodynamicflow conditions, this invention involves the use of materials having arelatively low mass per unit area of the leaflet materials. Also, thesecond moment of area of the cross-section and the bending modulus ofthe material are of minimal magnitude. To attain these properties, afabric of relatively small thickness is provided, subject to theattainment of an adequate level of load bearing capacity and filamentdurability.

For optimum performance the elastomeric sheet materials of the inventionare also fabricated according to a number of other geometrical criteria.These criteria provide not only the above-mentioned properties in termsof thickness, compliance and stress-strain characteristics, but also theproperties that relate to the promotion of desirable natural tissuelayering upon the sheet material. Thus the sheet material is formed toprovide a fibrous reticular face. Upon implantation, there is formedupon this face, because of its specific fibrous reticular nature, asatisfactory thin membranous deposit of endothelial cells, without anappreciable fibrous overgrowth of proteinaceous fibers such as collagen.

Suitable elastomeric sheet materials may comprise flat woven or braidedyarns forming distributed foramina of appropriate maximum lateralaperture dimensions. Such foramina may be defined by the spacing betweenyarns or by the spacing between fibers within each yarn, or by bothtypes of spacing. Other textile fabrication methods for producing areticular fibrous sheet may be employed, for example knitting, flocking,needling, tufting, spun bonding, etc.

Additional features of this invention comprise certain geometrical andstructural relationships which, when embodied in multifilamentelastomeric yarns and fabrics, provide sheet materials that closelyapproximate the properties of natural heart valve leaflets and othernatural tissues.

DRAWINGS

FIG. 1 shows the main frame of the preferred form of the heart valve.

FIG. 2 shows a fabric ribbon in the configuration formed by inserting itinto the main frame between the rod portions of its legs.

FIG. 3 shows the second frame.

FIG. 4 is a top plan view of the frame shown in FIG. 1.

FIG. 5 is a top plan view of the frame shown in FIG. 3.

FIG. 6 shows the partially fabricated heart valve with the fabricinserted into the main frame and cut open preparatory to cementingthereto.

FIG. 7 is a developed view of the partially constructed heart valve,corresponding to FIG. 6.

FIG. 8 is a cross-sectional view taken on line 8--8 in FIG. 7.

FIG. 9 illustrates a flat braided fabric pattern.

FIG. 10 is a stress-strain diagram comparing the characteristics ofnatural leaflet tissue, prior art woven synthetic implants, andelastomeric sheet material according to this invention.

FIG. 11 is a set of typical design graphs used for selection of theappropriate combinations of yarns per unit of fabric width, denier, andnumber of filaments per yarn, thus taking into account the filamentdensities and diameters to achieve the desired hole dimensions, that is,the aperture dimensions of the foramina.

DETAILED DESCRIPTION

FIGS. 1 to 8 show a preferred form of aortic trileaflet heart valvereplacement. Referring to FIG. 1, there is shown a main frame 22comprising a single length of 0.1 cm. diameter round polypropylene rodbent into a form having three mutually equidistant, generally parallellegs 24, 26 and 28, each leg comprising a pair of rod portions slightlyspaced apart, the rod portions being connected at one end and divergingat the other end. The diverging rod portions form three lobes 30, 32 and34. The connected ends of the rod portions in each pair form bights 36,38 and 40. FIG. 4 is a top plan view of the main frame 22.

A second frame 42 (FIGS. 3 and 5) is formed of a single length of 0.1cm. diameter round polypropylene rod bent into a form having three lobes44, 46 and 48 generally congruent with the lobes 30, 32 and 34 so as tofit in close contact therewith as shown in FIG. 7.

The assembly is started by threading a ribbon 50 of elastomeric sheetmaterial of the type hereinafter described, in this case a plain wovenelastomeric fabric, through the three pairs of rod portions so as toproduce the configuration shown in FIG. 2. The yarns of the fabric aremultifilament yarns. The frame 22 is shown in FIG. 1 in explodedrelation to FIG. 2 for clarity of illustration. The upper selvage has nouncut yarns and forms the free edges 52, 54 and 56 of valve leaflets.

Thus a double layer of the fabric is passed through each pair of rodportions forming one of the legs 24, 26 and 28. It is necessary toattach the fabric firmly to these legs, and also to the connecting lobes30, 32 and 34. To facilitate this attachment, the fabric is preferablycut lengthwise externally of each leg as shown in FIG. 6. Referring toFIG. 6, adhesive such as polyurethane dissolved in tetrahydrofuran isapplied to attach the fabric to each of the legs as follows. Flaps suchas 58 and 60 are spread apart and the adhesive is applied at theexternal point of juncture of the flaps where they enter between the rodportions, in a continuous line extending between points a and b. Theadhesive material reaches to the external surfaces of the frame bypenetration through the fabric flaps along this line; that is, theadhesive contacts the rod portions of the frame only on their outersurfaces. The leaflets comprise only those portions of the fabric on theinside of the frame, and these portions are not penetrated by theadhesive. Thus local stiffening and resultant flex failure caused bysuch adhesive penetration is avoided.

The above method of adhesive application also distributes the stressesof flexure evenly along the margins of the leaflets, and avoidsexcessive stress concentrations. These margins are permitted to moveupon each flexure over the rounded contours of the surfaces of the rodportions that are located on the inside of the frame, and that are notpenetrated by the adhesive.

The attachment of the fabric to the lobes 30, 32 and 34 is nextaccomplished by first placing the second frame 42 adjacent these lobeswith the fabric pieces passing therebetween as shown in FIGS. 7 and 8.Adhesive 61 is then applied through the fabric and to the surfaces ofboth the main frame 22 and the second frame 42, in a continuous lineextending between the points b of the respective legs and connectingthese three points. As in the previous step, the adhesive materialpreferably does not penetrate any portion of the leaflet material lyingwithin the main frame 22, and remains out of contact with blood passingthrough the valve.

The foregoing steps essentially complete the fabrication of the leafletportions of the valve. The remaining steps of fabrication are for thepurpose of facilitating the suturing of the prosthesis within the heart.The excess fabric available on the outside of the frame can be rolledand consolidated along the junction line between the main and secondframes to provide attachment points for stitches during surgicalinsertion.

The frame material is preferably polypropylene, although other materialshave also been employed with success. Polypropylene has excellent flexendurance and chemical stability, but is difficult to attach by adhesiveto other materials. To facilitate adhesion, the main and second frames22 and 42 may be encapsulated with polyurethane by multiple dip coating.The resulting encapsulated frame components have proved to demonstratethe desired characteristics of polypropylene without structural failuresor breakdowns at the adhesive junctures. Another preferred material ofconstruction for the frame is the cobalt alloy sold under the tradedesignation "Elgiloy".

Valves employing the elastomeric fabric described herein have beentested in an accelerated fatigue tester to assess their long-termendurance characteristics. Fatigue failures so induced in prior artleaflet materials have generally occurred in the region of greatestfabric flexure, that is, along a line in each leaflet that isperpendicular to its free edge and substantially equidistant between thecontiguous legs. The failures have generally occurred by breakdown ofthe filaments in the yarns running parallel to the free edge of theleaflets. As a means of providing greater fabric strength along thelast-mentioned lines, woven fabrics may be provided with a greaternumber of load-bearing yarns in this direction. However, there is alimit to the increase that is possible using a plain woven patternwithout seriously disturbing the geometry of the fabric interstices.

An alternative fabric construction pattern having improved strengthagainst such fatigue failure is illustrated in FIG. 9. The fabric shownis a flat braided ribbon 62 comprising 3 sets of elastomeric yarns,namely, a first diagonal set 64, a second diagonal set 66 and alongitudinal inlaid set 68. The yarns in each of the three sets arepreferably multifilament untwisted yarns. The ribbon 62 is braided on aconventional flat braiding machine. It will be noted that each selvagehas uncut yarns and one of those becomes the free edge of each leaflet.Thus fraying of the free edges of the leaflets is avoided as in theexample described above. In this embodiment both of the sets 64 and 66perform the load-bearing function of a single set of yarns in theearlier-described fabric. The result is that a greater number of yarnshave a significant component of load bearing capacity oriented parallelto the free edge.

The fabric 62 of FIG. 9 is preferably formed by braiding the yarn sets64 and 66 with inlaid longitudinal yarns 68 in a well-known manner, thusproducing a type of triaxial fabric. Such flat braided fabrics have anadditional advantage over conventionally woven fabrics, in that they areinherently highly extensible in the cross machine direction, that is, inthe direction perpendicular to the yarns 68. Such fabrics make itpossible to produce a two-way stretch characteristic.

In the foregoing description, woven and braided fabrics have beendescribed as embodied in replacement heart valve leaflets. However, manyof the attributes of these fabrics as well as other textile sheetmaterials within the scope of this invention which have similarproperties and are produced by such methods as knitting, flocking,needling, tufting, spun bonding, etc., make them ideally suited forother biomedical applications as well. For example, vascular prostheses,particularly those with small diameters, require a combination of goodstretch characteristics and inherent biological inertness. In this case,fabrics essentially similar to any of those described herein may bewoven, braided or otherwise fabricated in tubular form for use asconduits for flowing blood.

With either of the above tubular weave patterns, twoway stretchcharacteristics may be imparted to the vascular prosthetic devices. Theradial compliance is particularly useful in avoiding stiffnessmismatches at the boundaries between the existing artery and itssynthetic replacement, particularly in small diameter arteries. Acircumferential extension ratio of 1.5:1 is typically necessary in orderto provide the proper match of properties, and this ratio has beenprovided by the above-described tubular materials.

We turn next to a description of the preferred fabrics for use in theabove-described heart valve application and other prosthetic implantuses.

The preferred elastomers comprising the sheet materials of thisinvention are thermoplastic polyether esters prepared bytransesterification from terephthalic acid, polytetramethylene etherglycol and 1,4-butanediol. These copolyesters possess a two-phase domainstructure consisting of continuous and interpenetrating amorphous andcrystalline regions. The amorphous elastomeric polyalkylene etherterephthalate soft segments contribute the elastomeric character to thepolymer, whereas the crystalline tetramethylene terephthalate hardsegments serve as thermally reversible tie points which are capable ofholding the polymer chains together without the need for conventionalcovalent crosslinks. The synthesis of these copolymers is described inan article by G. K. Hoeschele, entitled "Segmented Polyether EsterCopolymers--A New Generation of High Performance ThermoplasticElastomers," appearing in Polymer Engineering and Science, December,1974, Vol. 14, No. 12. In the practice of this invention it is preferredto select those copolymers having relatively larger amounts of the softsegments as compared with the hard segments and specific examples testedhave included the copolymers sold under the trademark Hytrel 4056 by E.I. du Pont de Nemours & Company (hereinafter called "Hytrel"). Theseexhibit exceptional low temperature flexibility, and when fabricated asmultifilament yarns of suitable denier they can be woven or braided toproduce fabrics having the desired properties.

Other elastomers that can be similarly fabricated are within the purviewof the invention. These include (1) polybutylene terephthalate, (2) ablock polyester polyurethane copolymer sold under the trademark"Pellethane" by Upjohn Company, (3) thermoplastic silicone blockcopolymer, and (4) a thermoplastic polyester elastomer sold under thetrademark "Arnitel" by Akzo Plastics.

The foregoing elastomers are extruded as filaments using multipleorifice spinnerets in a conventional manner, and low-twist multifilamentyarns are formed. These yarns are then woven or braided to form theprosthetic fabric, or otherwise fabricated into textile sheet materialhaving the mechanical and structural properties hereinafter described.

The advantages of elastomeric materials include their relatively lowtensile moduli at low levels of stress, as shown by FIG. 10. This figureillustrates the tensile stress-strain characteristics of the naturalheart valve leaflet tissue and two synthetic yarn materials. Stress ismeasured in grams of tension force per centimeter of leaflet or fabricwidth and strain in percent of original length. Curve 69 represents thenatural leaflet characteristic in the circumferential direction. Curve70 represents the natural leaflet characteristic in the radialdirection. Curve 71 is representative of a fabric formed with anelastomer according to this invention and specifically represents afabric woven of Hytrel yarn. Curve 72 represents a fabric formed with apolyester, namely polyethylene terephthalate which has been microcrimpedafter weaving in the manner described in the above-mentioned copendingapplication Ser. No. 901,085. By suitable microcrimping the fabric ofcurve 72 may be made to exhibit a low initial modulus up to a strain ofabout 20 percent, above which the crimps are straightened and themodulus is much higher. Either of curves 71 or 72 shows a modulus whichis low enough at low levels of strain to perform satisfactorily in aheart valve prosthesis. However, as noted above, the low modulusproperties are achieved in different ways.

As noted above, the elastomeric materials of the invention may befabricated in any of several ways to form textile sheets having thedescribed properties. The following description, as applied to the plainorthogonally woven embodiments in a heart valve implant, isillustrative.

Thickness

An important criterion of the woven fabric is its thickness. Preferably,this should not exceed approximately 0.06 cm, the thickness of thenatural heart valve leaflet. In addition, if the fabric is composed ofyarns each having eight or more filaments, the level of twist imposes acriterion as shown by the following expression:

    4d≦t≦(2n).sup.1/2 d                          (1)

where "d" is the filament diameter or the minimum lateral dimension ofthe filament cross section where the latter is not circular but oval orotherwise of flattened shape, "t" is the fabric thickness, and "n" isthe number of filaments per yarn. As used throughout this description"thickness" refers to a dimension at right angles to the plane of thefabric. (A separate criterion imposing an upper limiting value 12d isexplained below under the heading "Flexural Rigidity".) The basis forexpression (1) as applied to twist may be understood by reference to thefollowing discussion.

The expression "4d" represents the minimum theoretical thickness of thewoven fabric of multifilament yarns. This is attained when the warp andfilling yarns are equally crimped during fabric production, as byweaving, and the yarns are sufficiently flattened by reason of having alow level of twist. The crimp here referred to is termed "structuralcrimp" and is distinguished from the crimp described in said applicationSer. No. 901,085, which results from compaction and shrinking operationson the woven fabric. When the structural crimp is thus evenlydistributed, the fabric thickness equals twice the thickness of a yarn;and the minimum theoretical thickness of a yarn having any degree oftwist is twice the diameter of a filament, as required to accommodatethe continuous filament relocation that is a necessary concomitant ofthe twisted structure.

On the other hand, if the yarns were highly twisted they would assume amore nearly circular shape, and the yarn thickness could be approximatedby assuming that its cross section is a square made up of n^(1/2) rowsof filaments with n^(1/2) filaments per row. In that case the fabricthickness would be approximated by the expression 2n^(1/2) d. However,by reducing the level of twist the fabric thickness can be reduced, andthe use of thin fabrics confers several benefits. The flexural rigidityof the fabric is reduced, as are the bending stresses and strains in thematerial; the fabric weight per unit area is also reduced, thusminimizing the inertia-controlled response time of the leaflet duringits opening and closing actions; and the diffusion of nutrient intosubsequent tissue deposits is hindered to a minimum extent. Moreover theuse of more-or-less balanced structural crimp is preferred because itgives a uniform surface contour and hence a tissue deposit with a moreuniform thickness. For these reasons it is desired to impose a limit onthe level of twist.

Defining the yarn cross-section "aspect ratio" as the ratio of the widthof the yarn to its thickness, experience has indicated that acceptablefabric geometries are obtained for aspect ratios greater than 2.0. Ayarn having this aspect ratio, comprising "n" filaments arranged in "a"rows has a thickness of "ad" and a width nd/a of twice that value, fromwhich it may be derived that for a fabric of balanced structural crimpconfiguration, the upper limiting fabric thickness would be (2n)^(1/2)d. In general, fabrics with thicknesses lying in the lower and centralportions of the range defined by expression (1) are preferred.

If there are fewer than eight filaments per yarn, including the case ofmonofilament yarns, expression (1) is not generally applied as acriterion because the level of twist in fabrics of balanced structuralcrimp is not important as long as the fabric thickness does notmaterially exceed the preferred face to face value of 0.06 cm previouslymentioned.

An example of a suitable fabric according to expression (1) is anorthogonally woven fabric of identical warp and filling Hytrel yarns,there being 30 filaments per yarn each filament of 20.6 micron diameter.According to expression (1), the lower and upper limiting thicknesses ofthe fabric are 82 and 160 microns, respectively. The actual measuredthickness of the given fabric is 157 microns.

Interfilament and Interyarn Hole Distribution

As described in said copending application, it is preferred to have thelateral dimensions of the foramina, holes or interstitial spacingsbetween the yarns, or between the yarns and filaments, in the range of20 to 40 microns. In particular, it is preferred that at least 50percent of the superficial area of at least one face of the fabriccontains a substantially uniform distribution of foramina having amaximum lateral aperture dimension of 40 microns. This imposes certainrequirements upon "N," defined as the number of yarns per cm. of widthin the fabric, as shown by the following expression: ##EQU1## in which"c" is defined as the average hole diameter and preferably lies in therange between 20×10⁻⁴ cm and 40×10⁻⁴ cm, "d" is the filament diameter,and "b" is the number of filaments per row in each yarn. The maximumvalue in equation (2) is determined by the case in which the onlyappreciable apertures through the fabric are the spaces between theyarns, the filaments in each yarn being in close side-by-siderelationship. The minimum value is determined by the case in which thefilaments of the yarns are separated to an extent sufficient to make theapertures between the yarns no greater than those between the respectivefilaments of each yarn.

When a fabric is to undergo fiber redistribution (spreading) in thefinishing processes, then the minimum limiting value in expression (2)can be used as a guide to the type of fabric structure that can bemanipulated from some starting configuration to the final desiredgeometry. When the fabric is intended for use in the as-wovenconsolidated configuration, with little or no filament redistribution,the maximum value can be used to give the specification of the wovenfabric. It is convenient in these latter cases to produce design graphsthat embody the analytical relationship, cast in a form suitable fordirect use. This is shown in FIG. 11, for example, which shows a seriesof graphs based on fabrics in which the yarns are in the consolidated,two layer configuration (a=2), with hole diameters of 30 microns, thefilament density being 1.4 gm/ml. Similar graphs can be constructed forother yarns and also for other weave pattern geometries.

The graphs shown in broken lines in FIG. 11 show, for yarns of 30, 60,90 and 120 denier, the value of "N" for each value of "n". The graphsshown in solid lines show, for yarns having 1, 1.5, 2, 3 and 4 denierper filament, the value "N" for each value of "n".

An example of a numerical calculation using the upper limit value inexpression (2) is given below for the Hytrel fabric described abovehaving 30 filaments per yarn, each filament of 20.6 microns. This fabricis designed to be used without any subsequent filament redistribution;therefore, all the effective interstitial holes are between yarns, whichare themselves organized into approximate three-layer configurations(a=3). In this case, b=30/3=10; d=20.6 microns, and "N", the designthreads per unit length for 30 micron holes is given by: ##EQU2## andfabric woven to this specification has a structure in which most of theinteryarn spaces are approximately 30 microns in extent.

Flexural Rigidity

It has been pointed out that flexural rigidity of the fabric should beheld to a minimum. A mathematical analysis of this property takes intoaccount the Young's modulus "E" of the material, the second moment ofarea "I_(f) " of the cross section of a fiber, and the effective numberof rows of fibers in the yarn. The general formula for the flexuralrigidity "G_(f) " of a single fiber is

    G.sub.f =EI.sub.f                                          (4)

For the case where the "n" fibers in a yarn are all completely free tomove within the cross section the value G_(f) in equation (4) multipliedby "n" would equal the flexural rigidity of a yarn. On the other hand,such complete freedom does not exist in a prosthetic implant because thefibers have a tissue deposited thereon and this may lead to a minimal tomaximal increase in fiber restraint and stiffening of the fabric. In theworst case, if the restraint on the fibers were complete such that theywere maintained at all times during bending in a configuration of "a"rows of "b" filaments per row with the neutral plane for bending beingat the innermost row, it may be derived that the stiffness would beincreased by the following factor f_(s) : ##EQU3## over the value givenabove for the case where the fibers are completely free. Expression (5)shows that rigidity increases rapidly with increases in "a". Thus if "a"were "3", the maximum stiffening effect that could be imposed in theoryupon the fibers would be approximately one order of magnitude over thecompletely free case. In practice, however, the tissue actually formeddoes not have the maximum stiffening effect, and it has been found thatthe value of "a" may be as high as six without an undesirable stiffeningeffect. As a more practical measure, it is preferred to establish as acriterion that the yarns have a minor axis (thickness) that is no morethan six times the average filament diameter "d", producing a maximumfabric thickness of 12d.

Reverting to the previously-given example of a Hytrel fabric having 30filaments per yarn, each filament of 20.6 microns, the measured fabricthickness of 157 microns is well under the limit of 12d=247.2 micronsestablished by the criterion for flexural rigidity.

Tensile Compliance

The "N" yarns per unit of width in the fabric, each yarn being composedof "n" filaments, must be such as to provide a tensile strength andmodulus approximating those of the natural valve leaflet, describedabove. This places restraints on the available material and techniques.One centimeter width of artificial leaflet material will contain Nnfilaments, using the nomenclature defined above, and will have aneffective cross-sectional area of (Nnπd²)/4 sq cm. If the tensilemodulus of the fiber material is E gm/cm² then the modulus of the fabricin gm/cm width will be E_(f) =(ENnπd²)/4. Most conventional textilematerials have tensile moduli that fall in the range 0.4 to 1.0×10⁶p.s.i., and are at least two orders of magnitude stiffer than isrequired to match the stiffness of the heart valve material at maximumstress, and several thousand times stiffer than the heart valve materialat low stress levels. There are two useful approaches to the realizationof low tensile modulus: in the first approach described in saidcopending application, excess filament length is introduced into thefabric in the form of crimp. In the second approach described in thepresent application, low-modulus elastomeric materials are used as thefiber material, and the matching is achieved by means of an overallreduction in the slope of the stress-strain characteristic. Usingelastomeric materials of inherently low modulus, no crimping except forstructural crimp resulting from fabric production itself is needed, andthe fabric is geometrically simpler to model.

Several low modulus elastomeric materials, identified above, have beenexamined for their mechanical suitability for the leaflet application.These were spun into yarns with various filament diameters and deniersand their tensile behavior was measured. Data on a selection of thesematerials is given in Table 1 below.

                  TABLE 1                                                         ______________________________________                                        TENSILE PROPERTIES OF LOW MODULUS                                             ELASTOMERIC YARNS                                                                                                   (5)                                                                           Initial                                                                       Yarn                                    (1)      (2)      (3)        (4)      Tensile                                 Fabric   Yarn     No. of     Filament Modulus                                 Material Denier   Filaments  Diameter,μ                                                                          (gm/den)                                ______________________________________                                        Pellethane                                                                             113      30         21.8     0.09                                    Silicone 105      30         23.8     1.14                                    Arnitel  81       30         18.7     4.44                                    Hytrel   91       30         20.4     0.70                                    Hytrel   91       30         19.5     0.70                                    Hytrel   71       30         16.8     0.63                                    PBT      210      30         26.8     6.9                                     PBT      62       30         16.0     7.4                                     ______________________________________                                    

In Table 1, "Silicone" refers to the thermoplastic silicone blockcopolymer previously identified, and "PBT" refers to polybutyleneterephthalate, also previously identified. The first listed Hytrelexample having a filament diameter of 20.4 microns is the same yarnincorporated in the fabric example previously discussed, having 30filaments per yarn, although the measurement in the fabric gave asomewhat higher filament diameter reading of 20.6 microns.

Fabrics woven to have desirable tissue reactions have holes thatgenerally fall within the range 20 to 40 microns, as stated above.Preferably, at least 50 percent of the superficial area of at least oneface of the sheet material contains a substantially uniform distributionof foramina having a maximum lateral aperture dimension of 40 microns.This requirement is satisfied for the yarns described in Table 1 byincorporating them into fabrics with a yarn density "N" of approximately40 yarns/cm.

The modulus of the natural leaflet material at the working load level isbetween approximately 1000 and 3600 gms/cm width. Any of the materialsin Table 1, and others with similar tensile properties, are capable ofbeing incorporated into fabrics that have both suitable geometricconfigurations and the proper tensile response, within a factor of twoof these values.

Table 2 lists actual measurements of three woven fabrics each using thefirst listed Hytrel yarn in Table 1. The fabrics differed somewhat inthe average number of yarns per centimeter and in processing, but allwere satisfactory for use in prostheses.

                  TABLE 2                                                         ______________________________________                                        TENSILE PROPERTIES OF WOVEN HYTREL FABRICS                                    Average Tensile Modulus                                                       Up to 150 gms/cm Width                                                        Warp             Filling                                                      Direction        Direction                                                    ______________________________________                                        0.7 × 10.sup.3                                                                           0.6 × 10.sup.3                                         0.5 × 10.sup.3                                                                           1.0 × 10.sup.3                                         0.4 × 10.sup.3                                                                           0.6 × 10.sup.3                                         ______________________________________                                    

Thus, either by suitable manipulation of the geometric form of thefilaments and yarns, or by proper choices of filament tensile modulus,or by a combination of these techniques, it is possible to producefabrics that have the desired combination of properties for heart valveleaflet applications, and also for other prosthetic and medicalapplications that demand approximately the same property combination. Ingeneral, a fabric having a modulus that does not exceed twice thecircumferential tensile modulus of the heart valve leaflet material of3600 grams/cm width, and is not less than half the radial tensilemodulus of 1000 gm/cm width at the working load level of 150 gm/cm widthwill be adequate, and the mechanics of the valve suggest that the lowerthe modulus at extremely low levels of strain the better the performancewill be. The preferred elastomeric multifilament yarns have an averagetensile modulus up to a strain of 10 percent in the range of 0.05 to 5.0grams per denier, the denier of the filaments being within the rangebetween 0.5 and 20.

What is claimed is:
 1. A frame system for a heart valve prosthesiscomprising:a first frame having at least three generally parallel legseach comprising a pair of rod portions connected at one end anddiverging at the other end as lobes respectively connecting with rodportions of others of said legs, the lobes forming an aperturetherebetween, said legs being adapted to receive the margin of a valveleaflet between the rod portions thereof, whereby said leaflet may besecured to two adjacent legs and the interconnecting lobe so as to havea free edge extending between said adjacent legs; and a second frameadapted to nest with said first frame comprising a rod formed in aclosed loop to be substantially congruent with each of saidinterconnecting lobes between the points of divergence at said otherends of each of said pairs of rod portions, whereby said leaflet may besecured to a lobe by passing it between said frames.
 2. A frame systemas set forth in claim 1 wherein said legs are equidistant.
 3. A heartvalve prosthesis comprising:a first frame having at least threegenerally parallel legs each comprising a pair of rod portions connectedat one end and diverging at the other end as lobes respectivelyconnecting with rod portions of others of said legs, the lobes formingan aperture therebetween, a plurality of flexible leaflets each insertedbetween the rod portions of two adjacent legs of said first frame,secured to said legs and the interconnecting lobe and having a free edgeextending between said adjacent legs, the free edges of said leafletsbeing deflectable into mutual contact for sealing said aperture, and asecond frame comprising a rod formed in a closed loop to besubstantially congruent with said interconnecting lobes between thepoints of divergence at said other ends of each of said pairs of rodportions, the leaflets being secured to the lobes by passing thembetween said frames.
 4. A heart valve prosthesis as set forth in claim 3wherein said leaflets are constituted of fabric.
 5. A heart valveprosthesis as set forth in claim 3 wherein each of said leafletsconsists essentially of a textile of filaments, each leaflet beingsealed to the frames externally of the aperture by adhesive appliedalong the lines of juncture of each pair of leaflets where they enterbetween each pair of rod portions, and along said lobes where eachleaflet passes between said frames.
 6. A heart valve prosthesis as setforth in claim 3 wherein said legs are equidistant.